1. Field of the Invention
The present invention relates to conductive particles and a method of preparing the same.
2. Description of the Related Art
In recent years, in the fields of electric and electronics materials, conductive particles have been applied to conductive materials such as liquid-crystal spacers, micro-device mounting conductive adhesives, anisotropic conductive adhesives, conductive connecting structures, etc. and put to practical use.
The conductive particles for use in such conductive materials are required to show excellent properties with regard to connection resistance and connection reliability. In the conductive particles, therefore, not only the control of particle diameter of the polymer particles serving as base particles but also the control of hardness and elastic characteristics of the particles, deformation recovery factor after compression displacement, etc. will be of importance.
Examples of the conductive particles thereto known include the following.
For example, JP-A 62-185749 and JP-A 1-225776 disclose conductive microspheres using polyphenylene sulfide particles, phenol resin particles or the like as base microspheres.
Such conductive microspheres using synthetic resin particles as base microspheres are poor in deformation recovery performance after compressive deformation. Therefore, when a compressive load exerted on electrodes in the process of connecting the electrodes to each other by use of the conductive microspheres is removed, slight gap would be formed at the interface between the conductive microsphere and the electrode surface, resulting in contact failure.
JP-B 5-19241 discloses conductive particles in which particles of a soft low-density cross-linked material composed mainly of styrene are used as base particles and their surfaces are coated with a conductive material.
However, such conductive particles having a soft base material also have a low deformation recovery factor of 10% or below after compressive deformation, and the restoring force is lowered with the lapse of time. Therefore, in the case where the conductive particles having the soft base material are used for connection between electrode, also, the connection resistance would increase with the lapse of time, leading to poor connection reliability.
JP-B 7-95165 and JP-A 2000-309715 disclose conductive particles obtained by use of base particles of which the compressive elastic modulus and compressive deformation recovery factor upon 10% displacement of particle diameter are controlled to predetermined values. In addition, JP-A 2003-313304 discloses conductive particles which are composed of a polymer obtained by using a divinylbenzene-ethylvinylbenzene mixture as part of the starting monomers and of which the compressive elastic modulus, compressive deformation recovery factor and breaking strain upon 10% displacement of particle diameter are controlled to predetermined values.
However, most of these conductive particles show a substantial breaking point at a compression displacement of less than 45%, and it is difficult with these conductive particles to enhance the deformation recovery factor upon a high compression displacement. Moreover, the base particles are susceptible to erosion by a strongly acidic or strongly alkaline medium in a plating step, which is a step for forming a conductive film on the base particles. As a result, in most cases, the physical properties intrinsic of the polymer particles would be lowered, and the exfoliation or cracking of the plating film would adversely affect the compressive elasticity characteristics of the base particles constituting the core parts of the conductive particles, resulting in a large lowering in physical properties such as hardness.
Meanwhile, technologies for forming a coat layer on the surfaces of resin particles so as to enhance the adhesion between the resin particles and a conductive material layer thereon are disclosed in JP-A 8-193186 and JP-A 2003-208813.
More specifically, JP-A 8-193186 discloses conductive particles for an anisotropic conductive adhesive in which insulating particles each composed of an inner core and an outer layer coating the inner core and softer than the inner core are further coated with a conductive material, whereas JP-A 2003-208813 discloses conductive particles in which resin particulates each composed of a core particulate and a resin coat layer formed on the surface of the core particulate are coated with a metallic coat layer.
However, in both the conductive particles disclosed in JP-A 8-193186 and JP-A 2003-208813, the core particles (polymer particles) are intrinsically poor in high compression displacement property, so that the high compression displacement amount and deformation recovery factor of the conductive particulates obtained are not so effectively improved; as a result, it is difficult to improve the connection resistance property and the connection reliability. In addition, when the outer layer is made thicker, the physical properties of the soft base material would play a greater role, like in the case of JP-B 5-19241, which adversely affects the physical properties indispensable to conductive particles, such as heat resistance and solvent resistance. Moreover, since the deformation recovery factor upon a high compression displacement is poor, the coating of the core particles with the outer layer is insufficient in the effect of enhancing the adhesion between the core particle and the conductive material layer.
Thus, there have not been known any conductive particles such that compressive deformation characteristics are not considerably lowered even upon formation of a conductive layer, and good deformation recovery performance after a high compression displacement is secured, so that the area of adhesion between the particles and an electrode surface can be enhanced.